1. Field of the Invention
The present invention relates to a method for treating a hydrocarbon oil, and more particularly to a method for upgrading a heavy oil feedstock by catalyst assisted hydrotreatment.
2. Description of the Related Art
Crude oils range widely in their composition and physical and chemical properties. In the last two decades the need to process heavier crude oils has increased. Heavy crudes are characterized by a relatively high viscosity and low API gravity (generally lower than 25.degree.) and high percentage of high boiling components. To facilitate processing, such heavy crudes or their fractions are generally subjected to thermal cracking or hydrocracking to convert the higher boiling fractions to lower boiling fractions, followed by hydrotreating to remove heteroatoms such as sulfur, nitrogen, oxygen and metallic impurities.
Acidic compounds, particularly naphthenic acids, are often found in crude oils. Naphthenic acids are carboxylic acids having a ring structure, usually of five member carbon rings, with side chains of varying length. Such acids are corrosive towards metals and must be removed, for example, by treatment with aqueous solutions of alkalis such as sodium hydroxide to form alkali naphthenates. However, with increasing molecular weight the alkali naphthenates become more difficult to separate because they become more soluble in the oil phase and are powerful emulsifiers.
The acidic content of a hydrocarbon oil is measured by the total acid number, or "TAN", which is defined as the milligrams of potassium hydroxide (KOH) necessary to neutralize the acid in 1 gram of oil. Typical refineries can process crudes having a TAN of up to 0.3. Some crude oils have TAN's of more than 4.0, making it difficult to process such oils.
What is needed is a process to upgrade heavy acidic hydrocarbon oils to simultaneously reduce acidity and increase API gravity. Moreover, an upgrading process operating at moderate pressures would be economical to set up and easy to operate.